All of the real estate electronic lock box systems in use today guard against unauthorized use by requiring the real estate agent to enter a personal identification code (PIN) at some point during the lock access process. PIN security is the weakest link in the overall device security since PIN's can be freely shared by individuals, or stolen by observation. PIN's can also be undesirable because the necessity of a fairly short number of digits to satisfy the convenience of memorization by the user leaves the number of possible PIN permutations small, and therefore, fairly easy to guess.